1. Field of the Invention
This invention relates to the amplification of optical beams using two-beam coupling, and more particularly optical amplification capable of producing a near diffraction limited output.
2. Description of the Related Art
Several techniques are currently available for extracting a near diffraction limited optical beam from an array of diode amplifiers. A diffracted limited beam is one with minimum dispersion and a generally planar wave front. The techniques are:
(1) Non-linear optical phase conjugation, in which an input beam is routed through an array of semiconductor amplifiers, phase conjugated, and then returned through the same semiconductor amplifiers for further amplification. This technique is disclosed in Stephens et al., "Phase conjugate master oscillator-power amplifier using BaTiO.sub.3 and AlGaAs semiconductor diode lasers", Applied Physics Letters, Vol. 50, pages 647-649 (1987). PA1 (2) Self-imaging or Talbot cavities, as disclosed in Ledger et al., "Coherent addition of AlGaAs lasers using microlasers and diffractive coupling", Applied Physics Letters, Vol. 52, pages 1771-1773 (1988). PA1 (3) The use of external prisms, as described in Carlson et al., "Coherent coupling of independent grading-surface-emitting diode laser array using an external prism", Applied Physics Letters, Vol. 56, pages 114-116 (1990). PA1 (4) The microfabrication of vertical cavities, as described in Yoo et al., "Fabrication of a two-dimensional phased array of vertical-cavity surface emitting lasers", Applied Physics Letters, Vol. 56, page 1198 (1990).
However, each of the above techniques suffer from various limitations. These include thermal loading due to the high injection current required to operate the system, a difficulty in the practical implementation of a high modulation rate required for optical communications unless external modulators are used, and an inability to achieve a true diffraction limited optical beam with a high average power.
No high power optical beam sources are currently available for certain wavelengths. For example, 1.55 microns is used for satellite communications and fiber optic systems, but a high power laser at this wavelength has not been developed. Distributed feedback semiconductor lasers have recently been developed that oscillate at 1.55 microns and are capable of tens of gigahertz modulation, with a flat FM response from 100 kHz to 15 GHz, Ogita et al., "FM response of narrow-linewidth multielectrode .lambda./4 shift DFB laser", Photonics Technology Letters, Vol. 2, page 165 (1990). While this type of laser is capable of high modulation rates and generates a near diffraction limited beam, it is restricted to low power levels.
An optical energy transfer system that uses resonant-two-beam coupling and can be used to amplify one beam at the expense of another beam has been discovered, and is described in Berman et al., Spectral Line Shapes, Vol. 3, F. Rostos ed., published by De Gruyter of Berlin, pages 337-339 (1985). While this technique has been shown to be capable of producing efficient energy transfers between two input beams in sodium vapor. It is not applicable to the high power amplification of beams at frequencies such as 1.55 microns. Two-beam coupling in a photorefractive material such as BaTiO.sub.3 has also been demonstrated, patent No. 4,761,059, issued Aug. 2, 1988 to Yeh et al. However, a coupling medium like BaTiO.sub.3 has a very slow response time, on the order of seconds, and can therefore be used only for essentially continuous wave applications. Due to the intrinsic limitation of BaTiO.sub.3, high power operation cannot be achieved.
A transfer of energy between beams of different wavelengths has been achieved by coupling beams within a host medium that has a rare earth dopant with an energy transition between the wavelengths of the two beams. This type of optical energy transfer mechanism has been demonstrated to have a high gain (30 dB) in the 1.5 micron regions Mears et al., "Low noise erbium-doped fiber amplifier operating at 1.54 .mu.m", Electronics Letters, Vol. 23, pages 1026-1028 (1987). Its application to erbium-doped optical fiber amplifiers is discussed in Desurvire et al., "Gain Saturation Effects in High-Speed, Multichannel Erbium-Doped Fiber Amplifiers at .lambda.=1.53 .mu.m", Journal of Lightwave Technology, Vol. 7, No. 12, December 1989, pages 2045-2104; Ainslie et al., "Erbium Doped Fibers For Efficient Optical Amplifiers", IEEE Proceedings, Vol. 137, Pt. J, No. 4, August 1990, pages 205-208; and Tachibana et al., "Erbium-Doped Fiber Amplifier With Flattened Gain Spectrum", IEEE Photonics Technology Letters, Vol. 3, No. 2, February 1991, pages 118-120.
While this technique is capable of achieving high modulation rates in the 10 GHz regime and a relatively flat. optical gain response, it does not preserve the diffraction limited quality of an input beam to which energy is transferred. Rather, an input beam that is originally diffraction limited will be distorted to a non-diffraction limited beam during the amplification process.